Process for converting magnesium fluoride to calcium fluoride

ABSTRACT

This invention is a process for the conversion of magnesium fluoride to calcium fluoride whereby magnesium fluoride is decomposed by heating in the presence of calcium carbonate, calcium oxide or calcium hydroxide. Magnesium fluoride is a by-product of the reduction of uranium tetrafluoride to form uranium metal and has no known commercial use thus its production creates a significant storage problem. The advantage of this invention is that the quality of calcium fluoride produced is sufficient to be used in the industrial manufacture of anhydrous hydrogen fluoride, steel mill flux or ceramic applications.

BACKGROUND OF THE INVENTION

The invention relates generally to a process for converting magnesium fluoride (MgF₂) to a useable product and more specifically to the conversion of MgF₂ to calcium fluoride (CaF₂) using calcium hydroxide (Ca(OH)₂), calcium oxide (CaO) or calcium carbonate (CaCO₃) and is a result of a contract with the United States Department of Energy.

In the reduction of uranium tetrafluoride to uranium metal with magnesium for use at various DOE facilities, large quantities of MgF₂ slag are produced. Presently this magnesium fluoride is processed through one of two routes depending on whether it contains enriched or depleted uranium. If the slag is produced from reduction of enriched uranium it is processed by leaching with nitric acid for uranium solubilization and recovery. The uranium barren slag is neutralized with lime, filtered, placed in drums and eventually dried for long term storage. Some of the magnesium fluoride, from both depleted and enriched uranium, is recycled as liner material for future reductions. The slag from depleted uranium production, when not recycled, is placed in below ground pit storage, but the procedure of burying materials can threaten underground water sources. It is preferred to minimize the amount of slag requiring storage, therefore a project was begun to develop a process converting the magnesium fluoride slag to usable, recyclable and possibly saleable material.

An attempt was made to use the MgF₂ to make anhydrous hydrogen fluoride (HF) but the reaction proved to be very slow and incomplete and resulted in a need for storage of voluminous quantities of magnesium sulfate contaminated with residual uranium. Since anhydrous HF is usually made from CaF₂, investigation were begun to study the possibility of converting MgF₂ to CaF₂ but the literature was not encouraging, Mellor's A Comprehensive Treatise on Inorganic and Theoretical Chemistry specifically states that CaO does not react with MgF₂ ; that MgF₂ is decomposed by alkali carbonates, not mentioning alkaline earth carbonates (such as CaCO₃); and is silent on the use of Ca(OH)₂ in such a process. Nevertheless work was begun to see if MgF₂ would be decomposed using CaO, CaCO₃ or Ca(OH)₂.

SUMMARY OF THE INVENTION

In view of the above-mentioned need, it is an object of this invention to provide a process for the conversion of MgF₂ to CaF₂.

It is another object of this invention to solve the problem of storage of large quantities of MgF₂.

It is further object of this invention to convert MgF₂ to a useable substance.

Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objects, the process is generally the conversion of MgF₂ to CaF₂ by mixing either CaO, Ca(OH)₂, or CaCO₃ with MgF₂ and heating the mixture to at least 1,600° F. for not less than two hours. The reactions that take place are:

    MgF.sub.2 (solid)+CaO(solid)→CaF.sub.2 (solid)+MgO(solid)

    MgF.sub.2 (solid)+Ca(OH).sub.2 (solid)→CaF.sub.2 (solid)+MgO(solid)+H.sub.2 O(gas)

    MgF.sub.2 (solid)+CaCO.sub.3 (solid)→CaF.sub.2 (solid)+MgO(solid)+CO.sub.2 (gas)

The solid by-product, MgO, is removed from CaF₂ using an acid leach and filtration to separate the solution from the solid CaF₂. This CaF₂ cake is dried to provide CaF₂ powder. The other by-products, CO₂ and H₂ O, are gases and naturally depart as such. Since HNO₃, HCl, H₂ SO₄ or any organic acid will work for the acid leach, the preferred acid depends on economic considerations and the by-products desired.

This method of converting MgF₂ to CaF₂ is advantageous because the resulting CaF₂ is useful in the manufacture of anhydrous HF whereas MgF₂ is of no known commercial use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is a process whereby MgF₂, particularly MgF₂ slag formed in the reduction of UF₄ with Mg, is converted to CaF₂ using either CaO, Ca(OH)₂ or CaCO₃. The three compounds were tested, each giving especially satisfactory results yielding better than 90% conversion from MgF₂ slag, so the choice of the calcium compound is discretionary.

The MgF₂ slag containing small amounts of uranium (2-2.5%), magnesium oxide (1-1.5%), and water (0.1-0.5%) is milled to increase its surface area and mixed with either calcium oxide, calcium hydroxide or calcium carbonate using stoichiometric amounts or slight excesses of the calcium containing compounds. This mixture is placed in a furnace and heated to at least 1,600° F. for two hours or more to form CaF₂ and the by-products. After the reaction is complete the by-products are separated from the CaF₂ by leaching with an acid solution. Sufficient acid must be used for stoichiometric reaction with the MgO and uranium which is present. The acid can be HCl, HNO₃, H₂ SO₄ or any organic acid. The chosen acid must be dilute enough to keep the resulting magnesium salts in solution. In tests, a 7-8 N HNO₃ solution was used with excellent results.

The acid solution containing the by-products is filtered from the solid CaF₂ leaving a CaF₂ cake that contains approximately 50% water. The cake is dried with heat, yielding a product of better than 90% CaF₂. The impurities are MgF₂ and less than 500 ppm uranium.

Since the literature stated that treatment of MgF₂ with CaO would not yield CaF₂ and also suggested that using CaCO₃ and Ca(OH)₂ would be unsuccessful, it was surprising that all three gave such impressive results.

The CaF₂ produced is of sufficient purity to be used in the industrial manufacture of anhydrous HF. This is a considerable advantage since the MgF₂ from which it is made has no use and creates a significant disposal or storage problems. The by-product MgO can be converted to Mg metal, however, known processes are not economically feasible under present industrial conditions. Although incomplete, procedures are being developed to convert the MgO to magnesium metal which could be used or perhaps sold, thus eliminating the burdensome storage problem.

The foregoing description of a preferred embodiment has been presented to explain the invention and enable others skilled in the art to best utilize it in a particular application. Obviously, many modifications are possible and one versed in the art will be able to determine the most suitable process conditions for any given set of circumstance without resorting to undue experimentation. 

We claim:
 1. A process for the conversion of MgF₂ to CaF₂ comprising:mixing MgF₂ with a stoichiometric amount or slight excess of a calcium-containing compound selected from the group CaO, Ca(OH)₂, and CaCO₃ ; heating said mixture to 1,600° F. or higher for two hours or longer to convert said magnesium fluoride and said calcium-containing compound to CaF₂ and by-products; leaching said CaF₂ and said by-products with an acid solution of sufficient concentration to dissolve said by-products; removing said acid and by-product solution from said CaF₂ resulting in wet CaF₂ that contains approximately 50% water; and drying said wet CaF₂ to produce CaF₂.
 2. The process of claim 1 wherein said acid is selected from the group HNO₃, HCl, H₂ SO₄ and all organic acids. 